--- a/platforms/debian7/Dockerfile.in	Wed Apr 08 21:59:47 2015 +0200
+++ b/platforms/debian7/Dockerfile.in	Tue Apr 14 23:36:09 2015 +0200
@@ -14,6 +14,7 @@
  libsqlite3-dev libldap2-dev libperl-dev autoconf \
  sudo net-tools \
  less
+RUN apt-get install --no-install-recommends -y libidn11-dev
 
 # prepare user environment for running (exim) and testing exim (eximtest)
 RUN useradd --system -c 'Exim User' exim